Promotion of plant growth using artificial light to augment or replace solar light has been the focus of significant research and experimentation. However, because each plant pigment absorbs light at one or more specific wavelengths, and the areas of peak absorption for each pigment are narrow, selecting and generating light wavelengths that produce superior plant growth remains a design challenge in grow light technology.
The absorption spectra of many plants containing chlorophyll exhibit peak absorption in the wavelength ranges of about 410-490 nanometers (“blue light”) and of about 590-650 nanometers (“red light”). Comparatively little light in the wavelength range of 500-570 nanometers (“green light”) is absorbed by the chlorophyll, which is consistent with the green appearance of plants. Consequently, conventional plant grow lights are designed to illuminate plants with peaks in both the blue light and red light regions corresponding generally to the plant absorption spectrum. Light in the wavelength range roughly centered on green light is often absent or near absent from the emissions of traditional grow lights. Of note is that the absorption spectra of most plants are also within the photopic spectral sensitivity for human vision, or “visible light,” generally defined as light having a wavelength in the range of about 400 to 700 nanometers (nm).
Lighting technologies such as incandescent, fluorescent, metal halide, and high-pressure sodium lamps have been employed in grow lights, with varying degrees of success. However, these conventional light sources suffer from various degrees of poor electrical efficiency (ranging from 10% to 40% of electrical energy consumed being converted to optical energy emitted within the visible spectral region) and low operation lifetimes (ranging from 1,000 hours to 24,000 hours of performance). Furthermore, these conventional light sources cannot control the spectral quality of emissions without the inefficient and limited utilization of additional filters. Moreover, these technologies' control of the radiation quantity is also limited, reducing the possibility of versatile lighting regimes such as pulsed operation. Also, the red-blue light of conventional plant grow lights is unattractive to humans as it appears purple and does not reveal the true or near true color of the plants that it illuminates.
Use of light-emitting diodes (LEDs) and related solid-state lighting (SSL) as potentially viable alternative lighting technologies for grow lights is gaining attention in the art. The best AlInGaP red and AlInGaN green and blue HB-LEDs can have internal quantum efficiencies better than 50%. Moreover, LED-based light sources support full controllability of both the direction and intensity of the emitted radiation, making it possible to avoid most of the losses associated with traditional grow lights. Additionally, the narrow spectral bandwidth characteristic of colored LEDs allows selection of the peak wavelength emission that most closely matches the absorption peak of a selected plant pigment. Furthermore, because LED lighting is much cooler than conventional plant lighting sources, an LED-based plant light may be placed much closer to a plant than a conventional plant light, with a resulting increase in light intensity falling on the leaves of a plant.
U.S. Pat. Nos. 5,278,432 and 5,012,609, both issued to Ignatius et al., disclose using an array of LEDs to provide radiant energy to plants broadly within bands 620 to 680 nm or 700 to 760 nm (red), and also 400 to 500 nm (blue). U.S. Pat. No. 6,921,182 to Anderson et al. discloses a proportion of twelve red LEDs (660 nm), plus six orange LEDs (612 nm) and one blue LED (470 nm). U.S. Patent Publication No. 2010/0259190 by Aikala discloses a single light emission source LED device capable of generating emission peaks that match well with a plant photosynthesis response spectrum. All of the implementations above attempt to provide an optimal mix of wavelengths to enhance plant growth, as well as low power consumption and long operation lifetime when compared to the existing grow light technologies. However, none of these implementations automatically adjust their emission spectra to match the specific energy absorption demands of different plants nor the evolving absorption demands of over time of a single plant. Furthermore, none of these implementations address the fact that the mix of wavelengths chosen to enhance plant growth are also unattractive to humans viewing the illuminated plant.
U.S. Patent Publication No. 2009/0199470 by Capen et al. discloses a lamp for growing plants that includes a set of red LEDs with a peak wavelength emission of about 660 nm, a set of orange LEDs with a peak wavelength emission of about 612 nm, and a set of blue LEDs with a peak wavelength emission of about 465 nm. The lamp also includes a green LED that has a wavelength emission between 500 and 600 nm, the purpose of which is to provide a human observer with an indication of general plant health. However, the disclosed implementation suffers from similar deficiencies as the references discussed above as it does not automatically adjust its emission spectra to match the specific energy absorption demands of different plants nor the evolving absorption demands of over time of a single plant. Furthermore, the course addition of green light to its mix of wavelengths falls short of revealing the true or near true color of the plants that it illuminates
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